A flexible line element, which comprises essentially a metal bellows, which is corrugated in a ring-shaped or helical pattern and through which an exhaust gas stream can flow, is known from DE 196 41 963 C1. The metal bellows may be lined with a flexible tube made of a braiding or knitted fabric on its inside facing the exhaust gas stream and/or on its outside facing away from the exhaust gas stream. The metal bellows may be connected upstream and/or downstream to a line section of the line element or of the exhaust system in the installed state.
Such a flexible line element is used within the exhaust system essentially to uncouple vibrations and may be arranged typically between a section of the exhaust system arranged in a fixed manner at the internal combustion engine and a section of the exhaust system arranged on the vehicle. Such a flexible line element may also be called uncoupling element. The flexible line element forms a part of the exhaust system in the installed state, so that the line element may also be called exhaust pipe element.
Modern exhaust systems may be equipped with an SCR system for reducing nitrogen oxides in the exhaust gas, where SCR denotes Selective Catalytic Reduction. Such an SCR system comprises a reducing agent feed means as well as an SCR catalytic converter downstream therefrom. A suitable reducing agent can be introduced into the exhaust gas stream by means of the reducing agent feed means. For example, ammonia is suitable for use as a reducing agent. However, urea or an aqueous urea solution is preferably introduced into the exhaust gas stream.
There is a risk under unfavorable boundary conditions or operating states of the internal combustion engine that urea and/or urea derivatives are formed in the exhaust gas stream, and these may become deposited on the walls of exhaust gas-carrying components. Such deposits of urea and/or urea derivatives can be regenerated, i.e., especially removed, dissolved or evaporated in favorable operating states. Unfavorable boundary conditions are, for example, low exhaust gas temperatures and low flow velocities in the exhaust gas. Insufficient mixing of the urea introduced with the exhaust gas stream may occur now. Furthermore, overdosage of urea is conceivable, in principle. Favorable boundary conditions are, by contrast, higher exhaust gas temperatures and higher flow velocities in the exhaust gas stream.
It may be desirable in exhaust systems with SCR system to install a flexible line element of the above-described type between the SCR catalytic converter and the reducing agent feed means. This is associated with the increased risk that the undesired deposits of urea derivatives will be formed precisely within the metal bellows. This applies especially to the case in which the metal bellows is exposed to the exhaust gas stream on its inside without an additional protective layer. The corrugated structure of the inside offers ideal possibilities for the deposition of urea and/or urea derivatives. It is problematic in this connection that the possibility of regeneration, i.e., also the possibility of evaporating or flushing out the urea and/or urea derivatives from the corrugated structure is significantly reduced, because direct admission with the exhaust gas stream is not usually possible there. What is formed in the area of the corrugated structure is a flow boundary layer, in which transverse motions hardly occur. However, the deposits in the metal bellows lead to the metal bellows becoming more rigid, as a result of which the function of the metal bellows as an uncoupling element is increasingly reduced.